The disclosed invention is directed generally to holograms for center high mounted stoplights for vehicles, and more particularly to a hologram that efficiently meets the required brightness and angular coverage and to an exposure technique for recording such hologram.
Present federal regulations require center high mounted stoplights (CHMSLs) in addition to the standard stoplights mounted in the rear portion of an automobile. The high mounted stoplights are intended to maximize the visibility of the automobile braking indicators to drivers following the braking vehicle, and are commonly mounted on the rear window of an automobile.
High mounted stoplights have commonly been implemented as a standard lenticular lens, a red filter, an illuminating incandescent bulb, and a reflector enclosed in a housing that is typically secured adjacent the top or bottom of an automobile rear window. However, the bulky housing partially obscures the rearward vision of the driver, and moreover imposes limitations on the design of the automobile.
Center high mounted stoplights have been also integrated into automobile body parts such as rear decks, spoilers, roofs, which to some degree substantially reduce or remove the rearward vision problem. However, such stoplights are complex and may impose limitations on the design of the automobile.
Holographic center high mounted stoplights have also been developed in order to efficiently meet the stoplight regulations. A consideration with holograms for holographic center high mounted stoplights are the luminous intensity and angular coverage requirements. Such requirements generally include a quantitative aspect that defines luminous intensity over a defined solid angular range, and a qualitative aspect that requires visibility over a horizontal angular range that is greater than the horizontal component of the defined solid angular range. Essentially, the regulations require a brighter region generally in the center of the angular region of coverage.
A straightforward procedure for recording a hologram for a center high mounted stoplight would be to record the image of a lenticular lens array that spreads light uniformly over a selected angular coverage (e.g., two lenticular arrays rotated 90 degrees with respect to each other and laminated together). A consideration with this procedure, however, includes the generation of spurious holograms since the lenses of the array would cover overlapping areas. Such spurious holograms reduce the efficiency of the desired stoplight hologram and moreover limit the see through clarity of the hologram. Also, precise control of the direction of the diffracted light is difficult.